Most conventional flow measurement products, i.e. flowmeters, are challenged for use in downhole oil and gas applications because of the rugged environment and handling during installation. The most common flowmeter used in oil and gas is based on differential pressure across some obstruction in the flow path or pipe such as an orifice plate, venture or jet cone. As an example, see Baker Hughes product bulletin entitled “SureFlo-InForm” at www.productionquest.com. These flowmeters have several shortcomings that limit their use. The mechanical restriction of these flowmeters is not desirable, and presents a liability and an obstacle for running intervention and logging tools. In addition, these systems use electronic pressure transducers which are limited in their reliability at higher temperatures which precludes their use in many deeper, hotter wells.
Fiber optic solutions have emerged to address these shortcomings. These products include distributed temperature sensors (DTS). As an example, see Sensa brochure entitled “Reservoir Surveillance in a Different Light” at www.sensa.org. Such products measure temperature along a fiber in or around the production tubing of the well. By monitoring temperature changes in time, flow can be inferred or modeled. See, for example, WellDynamics brochure entitled “iFlow Flow Profiling Analysis Service” at www.welldynamics.com.
Another fiber optic product is the only direct flow measurement product offered by Weatherford. This flowmeter is based on an array of optical sensors attached on the outside of a sensor “sub” pipe unit that tracks the velocity of flow-induced disturbances acting on the pipe. Volumetric flow can be calculated from the fluid velocity and pipe dimensions. Based on quartz optical fibers, these two fiber optic solutions are suitable at high temperatures to address most wells, and are non-obstructive in taking measurements on the outside of the production tubing. However, the DTS solution is not a direct measurement and therefore is subject to significant error in being inferred from temperature changes in which the quality of measurement is heavily dependent upon the quality of temperature measurement and validity of the model for given geological, thermal and reservoir characteristics of the formation. In contrast, the optical array sensor delivers high quality and reliable flow information, but requires complex optical interrogation equipment, including expensive modulation and receiver modules, and relatively complex processing electronics and software. In addition the construction of the transducer and sensing fiber packaging becomes quite demanding in the precision of fiber lengths and fiber mounting or coil winding which becomes a significant cost component of the system.
In view of the foregoing, there is an ongoing need for a simpler and more inexpensive approach to accurately obtaining information pertaining to fluid flow along a pipe.